The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process uses drilling equipment situated at surface with a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. In addition to this conventional drilling equipment, the system also relies on some sort of drilling fluid, in most cases a drilling “mud” which is pumped through the inside of the drill string, cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well away from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) a drill bit; 2) a steerable downhole mud motor of rotary steerable system; 3) sensors of survey equipment (Logging While Drilling (“LWD”) and/or Measurement-while-drilling (MWD)) to evaluate downhole conditions as well depth progresses; 4) equipment for telemetry of data to surface; and 5) other control mechanisms such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a metallic tubular.
As an example of a potential drilling activity, MWD equipment is used to provide downhole sensor and status information to surface in a near real-time mode while drilling. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, locations of existing wells, formation properties, and hydrocarbon size and location. This can include making intentional deviations from an originally-planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real time data during MWD allows for a relatively more economical and more efficient drilling operation.
In both directional and straight (or vertical) holes, the position of the well must be known with reasonable accuracy to ensure the correct well trajectory. While extending the wellbore, evaluation of physical properties such as pressure, temperature and the wellbore trajectory in three-dimensional space are important. The measurements include inclination from vertical and azimuth (compass heading). Measurements are typically made at discrete points with the general path of the wellbore computed from these points. In downhole MWD, the MWD tool surveys the well as it is drilled and information regarding the orientation of the drill bit is relayed back to the driller on surface. Measurement devices typically include a series of accelerometers which measure the inclination of the tool (for example vertical is 0° inclination and horizontal is 90° inclination) and magnetometers which measure the earth's magnetic field to determine azimuth. A typical Directional and Inclination (D&I) sensor package consists of three single axis accelerometers in each of the three orthogonal axes, together with two dual axes magnetometers yielding the three orthogonal axes and one redundant axis, which is typically not used. The sensor package also includes associated data acquisition and processing circuitry. The accelerometers and magnetometers are arranged in three mutually orthogonal directions, and measure the three mutually orthogonal components of the Earth's magnetic field and Earth's gravity. The accelerometer consists of a quartz crystal suspended in an electromagnetic field; measuring the inclination by how much electromagnetic force is required to maintain the crystal in balance. The accelerometers provide measurement of deviation from vertical, or inclination, as well as providing a measurement of the toolface or rotational orientation of the tool. The magnetometers provide a measure of the direction or magnetic heading as well as its orientation when the BHA is at or near vertical. These sets of measurements combined assist the driller for steering as well as for computing location. In most cases, whenever another length of drill pipe is added to the drill string, a survey is taken and the information is sent to surface and decoded by the MWD's operator and converted to information the driller requires for survey calculations. The BHA position is then calculated by assuming a certain trajectory between the surveying points.
In most downhole operations, it is often necessary to insert or introduce gauges, sensors or testing instrumentation into the borehole in order to obtain information of borehole parameters and conditions. Such parameters might include, but are not limited to, temperature, pressure, directional parameters, and gamma radiation. The electrical componentry of the various sensors and gauges used to obtain the information are mounted on or near circuit boards which are contained within an apparatus. The circuit boards may be referred or positionally favoured to one side of the carrier apparatus. The gauges are typically protected as they are imbedded in the wall, and hence completely housed, within the apparatus.
In MWD, known MWD tools contain essentially the same D&I sensor package to survey the well bore but the data may be sent back to surface by various telemetry methods. Such telemetry methods include, but are not limited to, the use of hardwired drill pipe, acoustic telemetry, fibre optic cable, Mud Pulse (MP) Telemetry and Electromagnetic (EM) Telemetry. In some downhole drilling operations there may be more than one telemetry system used to provide a backup system in case one of the wellbore telemetry systems fails or is otherwise unable to function properly. The sensors used in the MWD tools are usually located in an electronics probe or instrumentation assembly contained in a cylindrical cover or housing, located near the drill bit. The surface retrievable probe housing is subject to harsh downhole environments with increased temperature and pressure, excessive shock and vibration, as well as fluid harmonics which are created as drilling fluid passes by the probe. The electronics and sensors of the MWD tool can therefore be easily damaged.
In most current applications of directional drilling, standard D&I sensor packages contain fluxgate magnetometers for directional measurement and quartz flex accelerometers for inclination measurement; with the magnetometers and accelerometers taking measurements in each of the three orthogonal axes (X, Y and Z). In cases where any of the sensors fail, the tool must be removed for replacement or repair of the failed sensor as all sensors must be working for drilling to continue. Removal of the tool involves tripping out of hole, replacing the sensors or the whole MWD tool, and tripping back in which adds considerable time and increases well costs to the operator. The D&I sensors, and in particular the industry standard accelerometers, are a high cost component of the MWD tool and are prone to failure in the extreme environments they are subjected to.
MWD sensors are subjected to a number of internal and external influences which can cause errors associated with their use. Some internal influences include calibration errors, cross-axis sensitivity, temperature drift and output noise. External influences include BHA deflection, geomagnetic influence from magnetic ore in surrounding rock, drill-string induced interference, as well as axial misalignment. To offset some of the resulting effects, various calibration offsets are introduced into the sensor system. Typically the errors are assumed to be distributed normally among all three sensors in the orthogonal triad but this may not always be the case. Additionally, at higher temperature the sensors may deviate from calibrated values due to temperature drift, particularly when the sensors are also subjected to shock and vibration. The sensors may undergo temperature cycling as the probes are deployed and removed from the borehole. There can therefore be loss of calibration with time and use, which can lead to faulty measurements.
WO 2012/057055 describes the requirement to continually calibrate the magnetometers and accelerometers in the downhole survey instrument package to account for constant temperature drift and an apparatus for such calibration of the sensors. WO 2012/142566 relates to methods for calibrating logging measurements from a logging tool for which one of more attributes vary over the course of logging. The variable calibration function may be discrete or continuous and linear or non-linear. WO 2009/006077 describes a calibration method able to perform in situ calibration, analyze the calibration data and adjust at least one parameter based on the detected data. Each of these references is incorporated herein.